Automatic measurement and gain control of distributed Raman amplifiers

Abstract

Apparatus and method for gain measurement and control of a Distributed Raman Amplifier (DRA). Various embodiments of the apparatus include a detection unit operative to measure, during operation of the DRA, the optical power of a filtered component of the light entering the DRA from the transmission fiber and a gain calculation and control unit coupled to the detection unit and operative to calculate a signal Raman gain property from the measured optical power. The filtered component may exemplarily be a result of passing the light through a band pass filter, a spectral filter with a given spectral shape or a notch filter. The signal Raman gain property may be an average on-off signal Raman gain, an average net signal Raman gain or a signal Raman gain tilt within a communication band. The apparatus and method may be used to operate the DRA in Automatic Gain Control, i.e. to maintain a required constant signal Raman gain and/or signal Raman gain tilt.

The invention claimed is: 
     
       1. An apparatus for automatic gain measurement and control of a distributed Raman amplifier (DRA), the DRA providing Raman pump power at one or more pump wavelengths and being optically coupled to a transmission fiber which carries communication signals having wavelengths in a signal communication band, the apparatus comprising:
 a) a detection unit operative to filter light entering the DRA from the transmission fiber to provide a filtered component of light and to measure, during operation of the DRA, optical power of at least the filtered component of light; and, 
 b) a gain calculation and control (GCC) unit electrically coupled to the detection unit and operative to calculate a signal Raman gain property based only on the measured optical power. 
 
     
     
       2. The apparatus of  claim 1 , wherein the detection unit includes an optical supervisory channel (OSC) detector configured to measure the power of an OSC transmitted through the transmission fiber in an OSC band. 
     
     
       3. The apparatus of  claim 2 , wherein the detection unit further includes a signal detection sub-unit operative to measure the signal power and wherein the GCC unit is further operative to determine a change in the signal Raman gain property by analyzing simultaneous changes in the OSC power and the signal power. 
     
     
       4. The apparatus of  claim 1 , wherein the detection unit includes an amplified spontaneous emission (ASE) detector configured to measure the ASE power in a wavelength band different from that of the communication band and of an optical supervisory channel band. 
     
     
       5. The apparatus of  claim 1 , wherein the detection unit includes two amplified spontaneous emission (ASE) detectors configured to measure ASE powers within two ASE wavelength bands different from and located on opposite sides of the communication band. 
     
     
       6. A method for automatic gain measurement and control of a distributed Raman amplifier (DRA), the DRA providing Raman pump power at one or more pump wavelengths and being optically coupled to a transmission fiber which carries communication signals having wavelengths in a signal communication band, the method comprising:
 a) filtering light entering the DRA from the transmission fiber to provide a filtered component of light; 
 b) measuring during operation of the DRA, optical power of at least the filtered component of light; and 
 c) calculating a signal Raman gain property based only on the measured optical power. 
 
     
     
       7. The method of  claim 6 , further comprising the step of measuring, before operation of the DRA, the power of an optical supervisory channel (OSC) transmitted through the transmission fiber in an OSC band, wherein the filtered component includes OSC light in an OSC band, and wherein the step of calculating includes using the measurement before operation of the DRA and the measurement during operation of the DRA to calculate the signal Raman gain property. 
     
     
       8. The method of  claim 7 , further including the step of measuring the signal power in the communication band, and wherein the step of calculating further includes determining a change in the signal Raman gain property by analyzing simultaneous changes in the OSC power and the signal power. 
     
     
       9. The method of  claim 6 , wherein the filtered component includes ASE in an ASE wavelength band different from that of the communication band and of an optical supervisory channel band, wherein the measuring is performed at various levels of Raman pump power, and wherein the step of calculating includes using the results of the measurement of the ASE light optical power at various levels of Raman pump powers in the calculation of the signal Raman gain property. 
     
     
       10. The method of  claim 6 , wherein the filtered component includes ASE in an ASE wavelength band different from that of the communication band and of an optical supervisory channel band, and wherein the step of calculating includes determining a change in the signal Raman gain property by analyzing changes in the ASE power. 
     
     
       11. The method of  claim 6 , wherein the filtered component includes a first ASE component in a first ASE wavelength band different from and located on a first side of the communication band, and a second ASE component in a second ASE wavelength band different from and located on a second side of the communication band, wherein the optical powers of the first and second ASE components are measured separately, and wherein the step of calculating includes using the two ASE powers to calculate the signal Raman gain property. 
     
     
       12. An apparatus for automatic gain measurement and control of a distributed Raman amplifier (DRA), the DRA providing Raman pump power at one or more pump wavelengths and being optically coupled to a transmission fiber which carries communication signals having wavelengths in a signal communication band, the apparatus comprising:
 a) a detection unit operative to filter light entering the DRA from the transmission fiber to provide a filtered component of light and to measure, during operation of the DRA, optical power of at least the filtered component of light; and, 
 b) a gain calculation and control (GCC) unit electrically coupled to the detection unit and operative to calculate a signal Raman gain property based only on the measured optical power, wherein the signal Raman gain property is a property selected from the group consisting of a wavelength averaged on-off signal Raman gain, a wavelength averaged net signal Raman gain and a signal Raman gain tilt. 
 
     
     
       13. The apparatus of  claim 12 , wherein, based on the calculated signal Raman gain property, the GCC unit is further operative to control the pump power. 
     
     
       14. The apparatus of  claim 12 , wherein the detection unit includes a gain flattening filter (GFF) configured to spectrally flatten a signal Raman gain, a first signal detector configured to detect the optical power of the light before it passes through the GFF and a second signal detector configured to detect the optical power of the light after it passes through the GFF. 
     
     
       15. The apparatus of  claim 14 , wherein the GCC unit is further operative to calculate a net signal Raman gain by taking a first reading from the first signal detector before operation of the DRA, taking a second reading from the second signal detector during operation of the DRA, and calculating a difference between the second and the first readings. 
     
     
       16. The apparatus of  claim 12 , wherein the control of the pump power results in a required average signal Raman gain, while maintaining the signal Raman gain spectrally flat. 
     
     
       17. The apparatus of  claim 12 , wherein the DRA is part of a hybrid Raman/EDFA amplifier. 
     
     
       18. A method for automatic gain measurement and control of a distributed Raman amplifier (DRA), the DRA providing Raman pump power at one or more pump wavelengths and being optically coupled to a transmission fiber which carries communication signals having wavelengths in a signal communication band, the method comprising:
 a) filtering light entering the DRA from the transmission fiber to provide a filtered component of light; 
 b) measuring during operation of the DRA, optical power of at least the filtered component of light; and 
 c) based only on the measured optical power, calculating a signal Raman gain property selected from the group consisting of a wavelength averaged on-off signal Raman gain, a wavelength averaged net signal Raman gain and a signal Raman gain tilt. 
 
     
     
       19. The method of  claim 18 , further comprising:
 d) using the signal Raman gain property to control the pump power. 
 
     
     
       20. The method of  claim 18 , further comprising the step of measuring, before operation of the DRA, the optical power of the light entering the DRA from the transmission fiber, wherein the filtered component includes light passed through a gain flattening filter, and wherein the step of calculating includes using the measurement before operation of the DRA and the measurement during operation of the DRA to calculate the signal Raman gain property. 
     
     
       21. The method of  claim 18 , wherein the using the signal Raman gain property to control the pump power includes achieving a required average signal Raman gain within the communication band while maintaining the signal Raman gain spectrally flat.